South Korea has issued an urgent request to China to use its influence over North Korea to persuade the isolated nation to abandon its plans for a nuclear test.

Lim Sung-nam, a senior South Korean envoy, met with his Chinese counterpart on Monday as concern mounted that North Korea could carry out a nuclear detonation test this week.

China is North Korea's only real ally and widely seen as the only country with any leverage over Pyongyang.

Seoul's request to Beijing comes as South Korean and US naval forces begin a series of joint exercises in waters off the east coast of the Korean Peninsula.

The drills, in the Sea of Japan, were pre-planned, but are widely regarded as a tacit warning to North Korea.

Kim Jong-Un held a military meeting at the weekend

Speculation over an imminent nuclear test intensified over the weekend after reports in North Korean state media claimed that Kim Jong-Un, the country's young leader, had chaired an "enlarged meeting of his Central Military Commission". At the meeting, Mr Kim reportedly discussed a "looming great turn" in the country's military capability.

The English-language version of the report did not say when or where the meeting was held, but claimed senior military members of the commission were all present.

The state report read: "Kim Jong-Un made an important concluding speech, which serves as guidelines for further strengthening the (North's Korean People's Army) into a matchless revolutionary army."

Pyongyang has been threatening to conduct a nuclear test for some time.

However, plans for the test, the third since 2006, appear to have been accelerated since the latest round of UN sanctions against the country, themselves prompted by the surprisingly successful rocket launch by Pyongyang in December.

Officials in the South Korean capital Seoul have said their intelligence suggests that all the preparations have been completed and that Mr Kim could order the detonation at any time.

A tunnel under a nuclear test site (Pic: Yonhap News Agency)

The South Korean News agency has published an image which they claim shows the inner structure of a North Korean nuclear test site.

The image is said to be a screen shot from a documentary on North Korean television which shows a tunnel about one kilometre long with 10 doors.

Another image released by the Yonhap News Agency in Seoul shows a mocked-up cross-section of what the nuclear site could look like.

Last week, satellite imagery of the Punggye-ri nuclear test facility in North Hamgyong province seen by the South Korean intelligence officials suggested that covers had been placed over a tunnel entrance.

These latest tensions on the Korean Peninsula were sparked in December when North Korea defied and surprised the international community with the launch of a satellite into orbit.

The move prompted the United Nations to levy further sanctions on Pyongyang. The UN resolution had the unusual support of China.

Punggye-Ri nuclear test facility

The Chinese backing is being seen as a clear suggestion of Beijing's frustration at North Korea's continued defiance of international law.

An editorial in China's state-run Global Times newspaper on Monday suggested that China should seize the initiative over the North Korean problem.

"China has the largest stake in Asia, and it will be hit hardest if the situation in Asia becomes disordered," said the editorial; words which would have been sanctioned by the Chinese Communist Party machine.

"If China doesn't become more active in solving regional hot issues, it will inevitably be affected."

To date, most of Pyongyang's pronouncements have seen by Western governments as little more than bluster and rhetoric.

However, there is increasing international concern that North Korea's inexperienced leader plans to combine his two developing technologies - ballistic missile capability and nuclear detonation tests - to create a nuclear weapon.

Pyongyang's rockets already have the ability to reach far across East Asia, though suggestions that they could reach the west coast of the United States are thought to be wide of the mark.

Analysts also point out that there is a big technological gap between producing a rocket capable of firing a satellite into orbit and developing a nuclear warhead small enough to fit into a rocket which could then be fired accurately.

The concerns are compounded by the fact that reliable intelligence is significantly lacking.

Intelligence communities rely largely on the incomplete picture delivered through the many satellites passing over the peninsula.

Information from satellites is useful, but paradoxically, the key questions about how advanced Mr Kim's nuclear programme really is will only be answered once the third test has taken place.